1. Field of the Invention
The present invention relates to an articulated mechanism, and more particularly to a piping mechanism in an articulated mechanism such as an industrial robot or the like.
2. Description of the Prior Art
Hydraulically actuated articulated devices such as industrial robots or power shovels have hydraulically operated motors associated with respective articulations. For sending oil under pressure to and returning oil from each of the motors, it is necessary to employ two higher- and lower-pressure oil pipes. Since links interconnected by the articulations are movable relatively to each other, the pipes are usually in the form of flexible rubber hoses as disclosed in Japanese Laid-Open Patent Publication No. 57-48039, for example.
The flexible hoses are however expensive and require large joints on their opposite ends. A working space has to be provided around the links and articulations for assembling and maintaining the flexible hoses. Therefore, it is impossible to provide a compact arrangement around the motors in designing the articulated mechanisms. Moreover, the flexible hoses cannot neatly be arranged because of their nature, and always look disorderly as compared with rigid pipes of steel or the like. One mechanism which does not require any flexible hose is a swivel joint comprising a rotary body and rotatable oil seals juxtaposed axially on the surface of the rotary body, the oil seals defining spaces therebetween for use as a passage for a fluid under pressure. This mechanism permits oil under pressure to be transmitted through an articulation without employing any flexible hose.
One type of hydraulically actuated motor is an angular displacement motor or rotary actuator which is repeatedly angularly movable within a limited angular range. The angular displacement motor finds frequent use in various applications because it is of a simple construction and needs simpler inner seals than those used in a motor which produces continuously endless rotary motion. Japanese Laid-Open Patent Publication No. 55-51107 shows an example of such an angular displacement motor.
Hydraulic motors including angular displacement motors are generally small in size and capable of producing a high power output. Therefore, the pressure of a working fluid supplied to the hydraulic motor is selected to be as high as possible. However, the high fluid pressure applied to the motor exerts large forces to various parts of the motor, tending to deform these parts to the extent that smooth motor operation may no longer be available or tending to deform seals so that oil leakage is increased and hence the efficiency of the motor is lowered. One solution is to increase the rigidity of the parts of the motor, and hence the conventional motors and other hydraulic devices have been made of iron-base materials having high Young's moduli and had increased wall thickness. The motors and hydraulic devices thus constructed then make an entire hydraulic system heavy and have a large energy requirement. Another solution is to rely on the principles of a pressure balancing mechanism for applying a suitable counter pressure to the parts, which may be of reduced wall thickness, of a hydraulic motor to which the hydraulic pressure of working oil is usually applied. A variety of pressure balancing mechanisms have been proposed primarily in the field of hydraulic pumps.
A hydraulic motor is in principle the reverse of a hydraulic pump. It may be possible to employ a pressure balancing mechanism in a hydraulic motor for producing balancing forces. However, opposite pressures may not be balanced in an angular displacement motor because the angular displacement motor produces reciprocating angular motion. Typically, pressures acting on the opposite sides of a side plate of the angular displacement motor cannot be balanced well. More specifically, the side plate is apt to be pushed and expanded outwardly by oil pressure imposed on one side of the side plate from the working chamber in the motor. The oil pressure from the working chamber is not constant, but increases with the stroke of the motor. If the counterbalancing pressure applied to the other side of the side plate were set to a constant level, then the side plate would be excessively pushed back or not sufficiently pushed back as the motor stroke varies. If the counterbalancing pressure were relatively large, the side plate would be deformed when the working oil pressure from the working chamber is low. To avoid such side plate deformation, the thickness of the side plate must be increased. The working oil pressure applied from the working chamber to tend to deform the side plate varies with the motor stroke since an area for bearing the working oil pressure varies with the motor stroke. No appropriate counterbalancing pressure can be generated by providing a fixed area for bearing the counterbalancing pressure. Consequently, there has been a limitation on efforts to make the angular displacement motors lightweight.
The output torque of a hydraulically actuated angular displacement motor is greater as the radius of an angularly movable piston is larger. Stated otherwise, a region of the piston which is closer to the central axis thereof is less conductive to the generation of the output torque, and hence such a region is not an effective space for producing the output torque. Where a directional control valve for controlling the angular displacement motor is incorporated in the motor, the control valve is usually positioned at the central axis of the piston. If the axis of angular movement of an angular displacement motor is positioned substantially horizontally, then it is desirable that oil and air in the cylinder of the motor be well separated and air be discharged as rapidly as possible for the purpose of maintaining the response of a hydraulic system in which the motor is employed at a high level. To meet such a demand, a supply port for supplying oil under pressure to the cylinder should be positioned as high as possible. Heretofore, oil under pressure is supplied from the directional control valve at the central axis of the piston through a casing of the motor to a supply port which opens at the highest position on the cylinder. With such an arrangement, however, the oil passage extending from the directional control valve to the cylinder is long, making it impossible to make the response of the hydraulic system high. Moreover, the long oil passage requires a large space and designing the long oil passage is time-consuming and costly.